Currently, the heavy metal pollution of water bodies has become a severe worldwide challenge that attracts increasing concerns and researches throughout the world. In recent decades, the inorganic nanoabsorbents prepared with hydrated iron oxide (HFO) and hydrated manganese oxide (HMO) have been unanimously regarded as two of the best heavy metal absorbents. HFO presents unique amphoteric absorbability, namely, it can absorb not only the positively charged cations but also the negatively charged anions; its capability in absorbing arsenic, stibium is extraordinarily remarkable; a large amount of literature published both in China and in other countries has proven this fact. The researches on HMO date back to 1973. The comparatively large specific surface area, high activity and large absorption capacity of HMO particles enable it to present desirable absorbability for such heavy metals as lead, cadmium and zinc. However, despite the desirable properties of HFO and HMO mentioned above, there are many obstacles in putting them into practical application. Firstly, both HFO and HMO particles are extremely fine in size. When directly used in such a fluidization system as the absorption column, large pressure drop will occur; when used in a static absorption system, it would be difficult to guarantee an effective separation, and due to the existence of intraparticle diffusion, the absorption kinetics would not be satisfactory either. Secondly, due to the high activity and the thermodynamic instability of nanoparticles, severe agglomeration may occur in practical application, which will consequently lower down their absorption performance.
In recent decades, adopting polymer materials as the support and loading inorganic metallic functional materials thereon to prepare the composite absorbent has become a common practice, which effectively solves the above-mentioned problems. The materials often used as the support are resin, activated carbon, diatomite, zeolite, fibers and bentonite; all of them are good support materials, however, there still exist many problems requiring improvements.
Pan Bingcai and his team at (China) Nanjing University in two patents (Patent Number: ZL2005 1 00951775; ZL2006 1 004136654) disclosed a series of organic-inorganic composite nanoabsorbents by using nanoporous polymers as the support and then loading nanaparticles of iron oxide and manganese oxide into the nanoporous canals of the polymer support through surface deposition; the composite nanoabsorbents so obtained successfully solved the problem of the deep purification of water containing such pollutants as trace heavy metals, arsenic and selenium, because they not only overcame difficulties resulting from large pressure drop and separation problem when the oxide nanoparticles are directly applied in a fluidization system, but also greatly enhanced absorption selectivity for the target pollutants and absorption capacity as a whole due to the enhanced mass transfer effect engendered by the Donnan membrane formed by fixed charged groups on the surface of the polymer support.
Conventionally, the nanoporous canals of the support polymer are comparatively large (from tens to hundreds of nanometers); with so large the pore canals, the nanosize effect of the support is not very obvious; the size of loaded inorganic nanoparticles is slightly smaller than that of pore canals, but it is still as large as tens of nanometers or even more than one hundred nanometers. Such a structure leads to a series of defects, for example, poor nanosize effect, weak absorption activity, low absorption capacity, undesirable kinetic performance and low absorption selectivity for the target pollutant, for related researches have shown that the smaller inorganic functional particles, the stronger the nanosize effect and reaction activity, the higher absorption capacity, the more desirable kinetic performance and the higher absorption selectivity for the target pollutant.